Re: [RFC][2/3] Account and control virtual address space allocations

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Pavel Emelyanov wrote:
> Balbir Singh wrote:
>> Pavel Emelyanov wrote:
>>> Balbir Singh wrote:
>>>> Pavel Emelyanov wrote:
>>>>> [snip]
>>>>>
>>>>>> +int mem_cgroup_update_as(struct mm_struct *mm, long nr_pages)
>>>>>> +{
>>>>>> +	int ret = 0;
>>>>>> +	struct mem_cgroup *mem;
>>>>>> +	if (mem_cgroup_subsys.disabled)
>>>>>> +		return ret;
>>>>>> +
>>>>>> +	rcu_read_lock();
>>>>>> +	mem = rcu_dereference(mm->mem_cgroup);
>>>>>> +	css_get(&mem->css);
>>>>>> +	rcu_read_unlock();
>>>>>> +
>>>>>> +	if (nr_pages > 0) {
>>>>>> +		if (res_counter_charge(&mem->as_res, (nr_pages * PAGE_SIZE)))
>>>>>> +			ret = 1;
>>>>>> +	} else
>>>>>> +		res_counter_uncharge(&mem->as_res, (-nr_pages * PAGE_SIZE));
>>>>> No, please, no. Let's make two calls - mem_cgroup_charge_as and mem_cgroup_uncharge_as.
>>>>>
>>>>> [snip]
>>>>>
>>>> Yes, sure :)
>>> Thanks :)
>>>
>>>>>> @@ -1117,6 +1117,9 @@ munmap_back:
>>>>>>  		}
>>>>>>  	}
>>>>>>  
>>>>>> +	if (mem_cgroup_update_as(mm, len >> PAGE_SHIFT))
>>>>>> +		return -ENOMEM;
>>>>>> +
>>>>> Why not use existintg cap_vm_enough_memory and co?
>>>>>
>>>> I thought about it and almost used may_expand_vm(), but there is a slight catch
>>>> there. With cap_vm_enough_memory() or security_vm_enough_memory(), they are
>>>> called after total_vm has been calculated. In our case we need to keep the
>>>> cgroups equivalent of total_vm up to date, and we do this in mem_cgorup_update_as.
>>> So? What prevents us from using these hooks? :)
>> 1. We need to account total_vm usage of the task anyway. So why have two places,
>>    one for accounting and second for control?
> 
> We still have two of them even placing hooks in each place manually.
> 
> Besides, putting the mem_cgroup_(un)charge_as() in these vm hooks will
> 1. save the number of places to patch
> 2. help keeping memcgroup consistent in case someone adds more places
>    that expand tasks vm (arches, drivers) - in case we have our hooks
>    celled from inside vm ones, we won't have to patch more.
> 

I am not sure I understand your proposal. Without manually placing these hooks
how do we track

1. When the vm size has increased/decreased
2. In case due to some reason, the call following these hooks fail, how do we
   undo it, without placing hooks?


>> 2. These hooks are activated for conditionally invoked for vma's with VM_ACCOUNT
>>    set.
> 
> This is a good point against. But, wrt my previous comment, can we handle 
> this somehow?

Not sure I understand

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL

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